This invention relates to a surface-mount type crystal oscillator, for example, a crystal oscillator for use in a mobile communications apparatus.
A crystal oscillator of this type is an essential part for generating an oscillation signal for control of signal reception and transmission between mobile communications apparatus or the like. Such a crystal oscillator is required to have a very small volume as the mobile communications apparatus is constructed smaller and to have an oscillation signal having a stable frequency even if being used under environment where ambient temperature drastically changes.
In response to such request, temperature compensation is performed to make an oscillating frequency independent of ambient temperature against an intrinsic temperature-frequency variation characteristic of the crystal oscillator (for example, an AT-cut sliding crystal oscillator has a temperature-frequency variation characteristic represented by a three-dimensional curve). In order to conduct this temperature compensation in a small crystal oscillator with high accuracy, the variation in oscillating frequency of the crystal oscillator which varies with ambient temperature is flattened as a whole by setting a capacity value of a varicap diode at a suitable predetermined value based on temperature compensation data, using an oscillating inverter and an IC chip having integration of memory function for storing temperature compensation data corresponding to specified temperatures, voltage converting function, varicap diode function and control function.
Another crystal oscillator is known in which a crystal oscillating element is mounted in a main body as it is and an accommodation space therefor is hermetically sealed. A typical construction is as follows. A main body is formed with a cavity having a stepped portion. A control device is placed on a bottom surface of the cavity. A crystal oscillating element is placed on the stepped portion and the entire cavity is hermetically sealed by a metal cover. Such a construction does not use a can case and, therefore, can be made smaller because the crystal oscillating element and the control device are placed one on top of the other in the cavity along thickness direction. However, since the control device and the crystal oscillating element are arranged in the same cavity, the crystal oscillating element operates in an unstable manner if members for coupling or protecting the control device produce unnecessary gases.
Japanese Unexamined Patent Publication No. 10-28024 discloses a small crystal oscillator provided with external terminal electrodes on a surface thereof which satisfies the aforementioned demands and is able to highly accurately conduct a temperature compensation. This crystal oscillator includes a plate-like substrate and a rectangular hollow member attached on a bottom surface of the plate-like substrate. The rectangular hollow member has a rectangular space. The plate-like substrate and the rectangular hollow member defines a cavity. A crystal oscillating element is mounted on a top surface of the plate-like substrate while a control circuit is provided on a bottom surface of the plate-like substrate and in the cavity.
FIGS. 27 to 29 show a conventional temperature compensating crystal oscillator. This crystal oscillator is mainly comprised of a main body 1051, a rectangular crystal oscillating element 1052, an IC chip 1053 or controlling element constituting an oscillation control circuit and a metal cover 1054. This crystal oscillator includes a main body 1051 which is an integral assembly of a single-plate ceramic substrate 1055 and a rectangular hollow member 1056 provided on the bottom surface of the substrate 1055. The rectangular hollow member 1056 has a rectangular space. Thus, a cavity 1057 is defined in a lower portion of the main body 1051.
The ceramic substrate 1055 partitioning the top surface of the main body 1051 and the ceiling surface of the cavity 1057 is formed with viahole conductors 1058 in the thickness direction of the main body 1051 for electrically connecting the top surface side of the main body 1051 and the cavity 1057. A sealing conductive layer 1059 for sealing the metal cover 1054 is formed on the top surface of the ceramic substrate 1055. A wiring conductor 1060 including an IC electrode pad is formed on the ceiling surface of the cavity 1057. Further, external terminal electrodes 1061, 1062, 1063, 1064 are formed on the opposite longer sides of the bottom surface of the rectangular hollow member 1056. Four recesses extending up to the bottom surface of the rectangular hollow member 1056 are formed in the opposite shorter sides of the rectangular hollow member 1056, and terminal electrodes 1065 to 1068 are formed on the inner wall surfaces of these recesses. The terminal electrodes 1065 to 1068 are adapted to write temperature compensation data or other data in an IC (integrated circuit) chip 1053 mounted on the cavity 1057.
The rectangular crystal oscillating element 1052 is electrically coupled to the top surface of the main body 105 via mounts 1069 and 1070 using conductive resin adhesives 1071, 1072, and the metal cover 1054 substantially in the form of a dish is integrally coupled using the sealing conductive layer 1059 in order to hermetically seal the crystal oscillating element 1052. The IC chip 1053 is bonded to an IC electrode pad via a bump or a bonding wire. In the cavity 1057, resin 1073 is filled and cured, so that the IC chip 1053 is completely covered to have an improved resistance to humidity. In the aforementioned construction, the crystal oscillating element 1052 mounted on the top surface of the main body 1051 is connected with the IC chip 1053 via the viahole conductors 1058, and the IC chip 1053 is connected with the external terminal electrodes 1061 to 1064 and the temperature compensation data writing terminal electrodes 1065 to 1068 via the wiring conductors 1060. The IC chip 1053 and the terminal electrodes 1065 to 1068 are connected via the wiring conductor 1060 formed on a plane of the bottom surface side of the ceramic substrate 1055, and the IC chip 1053 and the external terminal electrodes 1061 to 1064 are connected by the viahole conductors 1058 extending through the thickness of the rectangular hollow member 1056, utilizing the inner wall surface of the hollow member 1056.
In the above crystal oscillator, the rectangular IC chip 1053 is used as a control circuit for controlling the oscillation of the crystal oscillating element. Thus, the cavity 1057 of the main body 1051 for accommodating the IC chip 1053 has a rectangular shape. The external terminal electrodes 1061 to 1064 connected to the IC chip 1053 are formed on the opposite longer sides of the bottom of the rectangular hollow member 1056.
However, it is very difficult to realize a stable operation of the crystal oscillator only by the aforementioned IC chip 1053. Specifically, high-frequency noise is likely to be added onto a power supply voltage supplied from an external terminal electrode, e.g., the VCC external terminal electrode 1061. Also, alternating-current components is likely to be added onto an output signal of an external terminal electrode, e.g., the external terminal electrode 1062. These noises can be removed by a large capacity capacitor, which is, however, difficult to integrate into the IC chip 1053.
In the conventional crystal oscillator, in order to avoid the oscillator becoming larger, a capacitor for performing the above operation needs to be mounted side by side with the crystal oscillator on a printed circuit board. However, this complicates the circuit construction of the printed circuit board, and results in increased labor and time to mount this capacitor. Also, an arrangement where electronic devices or chip capacitor, which serves as a capacitor element, are mounted side by side with the IC chip 1053 in the cavity 1057 having the rectangular space results in increase in the size of the cavity 1057. If the cavity 1057 were enlarged while the shape of the crystal oscillator in its top view is kept small, the external terminal electrodes 1061 to 1064 arranged around the cavity 1053 would have to be formed in smaller areas. This considerably reduces a bonding strength when the oscillator is soldered onto a printed circuit board.
In the crystal oscillator, the mounts 1069, 1070 are formed on the top surface, and the substrate having the electrode pad for bonding the IC chip 1053 formed on its bottom surface is constructed as the ceramic substrate 1055. Accordingly, the opposite ends of the viahole conductors 1058 extending along the thickness direction of the ceramic substrate 1055 are exposed to the space for accommodating the crystal oscillating element 1052 and to the ceiling surface of the cavity 1057. In other words, there is no problem if a portion between the ceramic substrate 1055 and the viahole conductors 1058, i.e., between the inner wall of through holes of the ceramic substrate 1055 and the outer circumferential surfaces of the viahole conductors 1058, is completely in close contact. However, this portion may be cracked if thermal coefficient of shrinkage of the ceramic substrate 1055 differs from that of the viahole conductors 1058 or the ceramic substrate 1055 may not necessarily be completely in close contact with the viahole conductors 1058. Even if this portion had been completely airtight before the crystal oscillating element 1052 and the IC chip 1053 were mounted, hermetic reliability is considerably reduced as a result of a plurality of heat treatments, such as during the mounting of the crystal oscillating element 1052, during the stabilization of the frequency of the crystal oscillating element 1052, during the mounting of the IC chip 1053 and during the filling and curing of the resin 1073. Thus, there is a likelihood that the airtight space for accommodating the crystal oscillating element 1052 communicates with the cavity 1057 via cracks formed around the viahole conductor 1058. As a result, the environment of the accommodation space enclosing the crystal oscillating element 1052 changes, thereby changing the oscillation characteristic.
An essential point in conventional construction in which the crystal oscillating element 1052 and the IC chip 1053 are mounted in different accommodation cavities to avoid an increase in height. Japanese Unexamined Patent Publication No. 10-28024 proposes a construction with which strength will not be reduced even if the metal cover is thinned. In other words, a dish-shaped metal cover formed with projections is used.
In keeping the height of the crystal oscillator low, a support construction for the crystal oscillating element mounted on the surface of the ceramic substrate 1055 is essential. A construction having a lowest height has such that crystal oscillating element supports 1069, 1070 are defined by crystal oscillating electrode pads, and the crystal oscillating element 1052 is mounted on the electrode pads using conductive resin adhesive 1071, 1072 including a cap member. With this construction, a spacing Between the top surface of the substrate 1055 and the crystal oscillating element 1052 corresponds to substantially the height of the cap member and a spacing Between the top surface of the crystal oscillating element 1052 and the metal cover 1054 can be defined by the shape of the metal cover.
However, in the case where the crystal oscillating element 1052 is mounted on the oscillating element electrode pad by the conductive resin adhesive 1071, 1072, conductive resin adhesive fed by a dispenser is liable to spread planarly. The spread conductive resin adhesive may come into contact with a sealing conductive layer which is arranged on the periphery of the top surface of the substrate 1055, resulting in unstable oscillation frequencies. This sealing conductive layer 1059 is adapted to couple the metal cover 1054 onto the substrate 1055. Accordingly, the use of conductive resin adhesive to couple them together is obvious since the sealing conductive layer 1059 and the element electrode pad are formed on the top surface of the substrate 1055 in proximity to each other to make the crystal oscillator smaller in its top view.
Further, it has been very difficult to couple a chip-shaped electronic device having a plane size of 1 mm.times.0.5 mm in the cavity 1057 of the miniaturized main body 1051 by soldering. Due to the miniaturization of the main body 1051, specifically, such an electronic device is accommodated in the same cavity 1057 in very proximity to each other. Thus, flux contained in solder paste for bonding the electronic device may deposit on the IC electrode pad or the wiring conductor 1060, thereby reducing the bonding reliability of the IC chip 1053 and the IC electrode pad 1060, or causing a short circuit due to solder balls. The use of conductive resin adhesive may be considered instead of soldering. Although conductive resin adhesive advantageously solves the aforementioned problems brought by the soldering, there is no sufficient self-alignment resulting from a surface tension which acts during the fusion of solder. As a result, it becomes very difficult to mount the chip-shaped electronic device in a specified tiny portion of the cavity 1057. Simultaneously, the conductive resin adhesive is likely to spread between a pair of electrode pads, resulting in a short between the pair of electrodes.
There has been known a construction in which an IC chip 1053 is mounted in a cavity 1057 formed in a lower portion of a main body 1051 by face-down bonding method using bumps, and resin is filled between the IC chip and the bottom surface of the cavity 1057. FIG. 30 shows a temperature compensating crystal oscillator mounted with an IC chip for performing a temperature compensation for a crystal oscillating element. In this crystal oscillator, an IC chip 1214 formed with gold bumps 1213 is accommodated in a cavity 1212 of a main body 1211 formed by placing a multitude of ceramic layers one on top of another, so that the gold bumps 1213 are in contact with electrode pads (not shown) on the ceiling surface of the cavity 1212. The IC chip 1214 is bonded to the bottom surface of the cavity 1212 of the main body 1211 via the gold bumps 1213 and the electrode pads by adhesion using silver paste or ultrasonic fusion. However, the bonding strength is so low that the IC chip 1214 is peeled off by the weight of 6 g per bump if it is bonded only by silver-paste adhesion or ultrasonic fusion. Accordingly, a thermosetting resin 1215 having a high coefficient of shrinkage which is called underfill resin is filled between the IC chip 1214 and the ceiling surface of the cavity 1212. As the resin is cured, the IC chip 1214 is fixed to the ceiling surface of the cavity 1212. Although unillustrated, the crystal oscillating element is mounted on the surface of the main body 1211 opposite from the cavity 1212 and hermetically sealed.
As shown in FIG. 31, the thermosetting resin 1215 contracts in directions of arrows in the conventional crystal oscillator. This contraction produces a stress F in a direction to pull the IC chip 1214 toward the ceiling surface of the cavity 1212, and the IC chip 1214 is fixed to the ceiling surface of the cavity 1212 only by this force.
In a crystal oscillator shown in FIG. 27, resin 1073 is likely to deposit on a wall surface defining a space of a cavity 1057 when the resin 1073 is filled into the cavity 1057 having the IC chip 1053 bonded therein. Unless a careful attention is paid, the resin 1073 may come out of the cavity 1057. For example, if a resin having excellent adhesiveness is filled to form a bottom layer and a resin having excellent resistance to humidity is filled to form a top layer in order to increase the bonding strength between the IC chip 1053 and the cavity 1057, the resin filled to form the top layer frequently comes out through the opening of the cavity 1057 to deposit on the outer surface around the cavity 1057. If the resin coming out of the cavity 1057 deposits on the outer surface around the cavity 1057, the crystal oscillator becomes defective due to its height deviated from standards. Further, the resin deposits on the external terminal electrodes 1061 to 1064 around the cavity 1057, resulting in defective connection between the crystal oscillator and the printed circuit board.